Microbiology Unit 2: Metabolism of Prokaryotes PDF

Unit 2: Metabolism of Prokaryotes *Bacteria - Growth curve and kinetics. * Quantification of bacterial growth * Microbial metabolism: Non-biosynthetic pathway, Biosynthetic pathway. * Adaptation mechanism of microbes: Halophiles, Alkalophiles, Psychrophiles, Piezophiles, Xeroph...

Unit 2: Metabolism of Prokaryotes *Bacteria - Growth curve and kinetics. * Quantification of bacterial growth * Microbial metabolism: Non-biosynthetic pathway, Biosynthetic pathway. * Adaptation mechanism of microbes: Halophiles, Alkalophiles, Psychrophiles, Piezophiles, Xerophiles. * Bacterial Recombination: Transformation, Transduction, Conjugation  BACTERIAL GROWTH In microbiology, growth is defined as an increase in the number of cells. Microbial cells have a finite life span, and a species is maintained only as a result of continued growth of its population. As macromolecules accumulate in the cytoplasm of a cell, they assemble into major cell structures, such as the cell wall, cytoplasmic membrane, flagella, ribosomes, enzyme complexes, and so on, eventually leading to the process of cell division itself. when one cell eventually separates to form two cells, we say that one generation has occurred, and the time required for this process is called the generation time  Bacterial Cell Cycles Can Be Divided into Three Phases The cell cycle is the complete sequence of events extending from formation of a new cell through the next division. The bacterial cell cycle consists of three phases: (1) a period of growth after the cell is born, which is similar to the G1 phase of the eukaryotic cell cycle; (2) chromosome replication and partitioning period, which functionally corresponds to the S and mitosis events of the M phase of the eukaryotic cycle; and (3) cytokinesis, during which a septum and daughter cells are formed GROWTH IN COCCI AND BACILLI FtsZ is the major cytoskeletal protein in the bacterial cytokinesis machine. It forms a ring (the Z ring) under the membrane at the center of the cell, and this Z ring constricts to initiate division of the cell. MreB is a protein found in bacteria that has been identified as a homologue of actin. MreB controls the width of rod- shaped bacteria.  GROWTH IN VIBRIO The last cell shape we consider is that of comma-shaped cells, as studied in the aquatic bacterium Caulobacter crescentus. In addition to the actin homologue MreB and the tubulin-like protein FtsZ, these cells (and other vibroid-shaped cells) produce a cytoskeletal protein called crescentin, a homologue of eukaryotic intermediate filaments. This protein localizes to one side of the cell, where it slows the insertion of new peptidoglycan units into the peptidoglycan sacculus. The resulting asymmetric cell wall growth gives rise to the inner curvature that characterizes the comma shape.  GROWTH CURVE OF BACTERIA Population growth is often studied by analyzing the growth of microbes in liquid (broth) culture. When microorganisms are cultivated in broth, they usually are grown in a batch culture; that is, they are incubated in a closed culture vessel like a test tube or a flask with a single batch of medium. Fresh medium is not provided during incubation, so as nutrients are consumed, their concentrations decline, and wastes accumulate. Population growth of microbes reproducing by binary fission in a batch culture can be plotted as the logarithm of the number of viable cells versus the incubation time. The resulting curve has five distinct phases: Lag Phase Exponential Phase Stationary Phase Death Phase Long-Term Stationary Phase  LAG PHASE When microorganisms are introduced into fresh culture medium, usually no immediate increase in cell number occurs. This period is called the lag phase. It is not a time of inactivity; rather cells are synthesizing new components. This can be necessary for a variety of reasons. The cells may be old and depleted of ATP, essential cofactors, and ribosomes; these must be synthesized before growth can begin. The medium may be different from the one the microorganism was growing in previously. In this case, new enzymes are needed to use different nutrients. Eventually, however, the cells begin to replicate their DNA, increase in mass, and divide. As a result, the number of cells in the population begins to increase.  EXPONENTIAL PHASE During the exponential phase, microorganisms grow and divide at the maximal rate possible given their genetic potential, the nature of the medium, and the environmental conditions. Their rate of growth is constant during the exponential phase; that is, they are completing the cell cycle and doubling in number at regular intervals The population is most uniform in terms of chemical and physiological properties during this phase; therefore exponential phase cultures are usually used in biochemical and physiological studies. The growth rate during exponential phase depends on several factors, including nutrient availability. When microbial growth is limited by the low concentration of a required nutrient, the final net growth or yield of cells increases with the initial amount of the limiting nutrient present (figure 7.11a). The rate of growth also increases with nutrient concentration (figure 7.11b) but it saturates, much like what is seen with many enzymes. The shape of the curve is thought to reflect the rate of nutrient uptake by microbial transport proteins. At sufficiently high nutrient levels, the transport systems are saturated, and the growth rate does not rise further with increasing nutrient concentration  STATIONARY PHASE In a closed system such as a batch culture, population growth eventually ceases and the growth curve becomes horizontal. Final population size depends on nutrient availability and other factors, as well as the type of microorganism. In stationary phase, the total number of viable microorganisms remains constant. This may result from a balance between cell division and cell death, or the population may simply cease to divide but remain metabolically active. Microbes enter the stationary phase for many reasons. One important reason is nutrient limitation; if an essential nutrient is severely depleted, population growth will slow and eventually stop. Aerobic organisms often are limited by O2 availability. Population growth also may cease due to the accumulation of toxic waste products. DnaA, the protein that binds to the chromosome’s origin to initiate replication, becomes less active in stationary phase.  DEATH PHASE Cells growing in batch culture cannot remain in stationary phase indefinitely. Eventually they enter a phase known as the death phase. During this phase, the number of viable cells declines exponentially, with cells dying at a constant rate. Detrimental environmental changes such as nutrient deprivation and the buildup of toxic wastes cause irreparable harm to the cells.  LONG-TERM STATIONARY PHASE Long-term growth experiments reveal that after a period of exponential death some microbes have a long period where the population size remains more or less constant. This long term stationary phase (also called extended stationary phase) can last months to years. During this time, the bacterial population continually evolves so that actively reproducing cells are those best able to use the nutrients released by their dying brethren and best able to tolerate the accumulated toxins. VBNC - viable but non-culturable state This dynamic process is marked by successive waves of genetically distinct GASP - Growth Advantage in Stationary Phase (GASP) SCDI - stationary phase contact-dependent inhibition variants. CASP - constant activity stationary phase Thus natural selection can be witnessed within a single culture vessel. Ref: Front. Microbiol., 16 October 2017 Sec. Evolutionary and Genomic Microbiology Volume 8 - 2017 | https://doi.org/10.3389/fmicb.2017.02000